1. Field of the Invention
The present invention relates to a low-pass filter and a mobile communication device using the same, and more particularly, to a low-pass filter used in a GHz band and a mobile communication device including the same.
2. Description of the Related Art
As known in the art, a low-pass filter operates to pass signals having a frequency lower than a desired frequency to remove a desired level, or higher, of harmonic signals, and includes an inductor and a capacitor.
FIG. 4 is a circuit diagram of a typical low-pass filter. The low-pass filter includes an LC parallel resonance circuit having an inductor L and an ungrounded capacitor C1 connected between an input terminal Pi and an output terminal Po. The low-pass filter further includes grounded capacitors C2 and C3 connected between the ends of the LC parallel resonance circuit and ground, respectively.
FIG. 5 is an exploded perspective view showing a conventional low-pass filter 50. The low-pass filter 50 includes a laminate defined by six dielectric layers 51a to 51f, as disclosed in Japanese Unexamined Patent Application Publication No. 7-336176. The dielectric layer 51a is a protective layer, the dielectric layer 51b has an inductor electrode 52 provided thereon, and the dielectric layers 51c and 51d have capacitor electrodes 53a and 53b provided thereon, respectively. The dielectric layer 51e has capacitor electrodes 53c and 53d provided thereon, and the dielectric layer 51f has a ground electrode 54 provided thereon.
One end of the inductor electrode 52 extends to one shorter side of the dielectric layer 51b and connects to the input terminal Pi, and the other end of the inductor electrode 52 extends to the other shorter side of the dielectric layer 51b and connects to the output terminal Po. One end of the capacitor electrode 53a extends to one shorter side of the dielectric layer 51c, and one end of the capacitor electrode 53c extends to one shorter side of the dielectric layer 51e to connect to the input terminal Pi. One end of the capacitor electrode 53b extends to one shorter side of the dielectric layer 51d, and one end of the capacitor electrode 53d extends to the other shorter side of the dielectric layer 51e to connect to the output terminal Po. Portions of the ground electrode 54 extend to longer opposing sides of the dielectric layer 51f to connect to the ground terminals Pg.
The inductor electrode 52 defines the inductor L, and the inductance of the inductor L depends upon the length of the inductor electrode 52. The capacitor electrodes 53a and 53b define the ungrounded capacitor C1, and the capacitance of the ungrounded capacitor C1 depends upon the spacing between the capacitor electrodes 53a and 53b, the overlapping area thereof, and the permittivity of the dielectric layer 51c. The capacitor electrodes 53c and 53d, and the ground electrode 54 define the grounded capacitors C2 and C3, and the capacitances of the grounded capacitors C2 and C3 depend upon the spacing between the capacitor electrode 53c and the ground electrode 54, the spacing between the capacitor electrodes 53d and the ground electrode 54, the overlapping area of each of the capacitor electrodes 53c and 53d with the ground electrode 54, and the permittivity of the dielectric layer 51e. 
However, the conventional low-pass filter encounters a problem. Specifically, in such a low-pass filter, a capacitor electrode that defines an ungrounded capacitor overlaps, via a dielectric layer, a capacitor electrode and a ground electrode that together define a grounded capacitor. This arrangement produces a capacitance between the capacitor electrode defining the ungrounded capacitor and the capacitor electrode defining the grounded capacitor, and also produces a capacitance between the capacitor defining the ungrounded capacitor and the ground electrode defining the grounded capacitor. Hence, the capacitances of the ungrounded capacitor and the grounded capacitor are not independently controlled, making it difficult to accurately control the pass band of the low-pass filter.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a low-pass filter which facilitates control of the pass band with improved insertion-loss and attenuation characteristics, and a mobile communication device using the same.
A low-pass filter includes a laminate containing an inductor electrode, a plurality of capacitor electrodes, and at least two ground electrodes. The laminate is defined by laminating a plurality of dielectric layers. The low-pass filter includes a grounded capacitor defined by a capacitor electrode and at least two ground electrodes, the capacitor electrode being located between the at least two ground electrodes, and an ungrounded capacitor including a capacitor electrode which is not located between the ground electrodes.
The inductor electrode is provided between the capacitor electrode, which defines the ungrounded capacitor, and the principal plane of the laminate.
According to another preferred embodiment of the present invention, a low-pass filter includes an LC parallel resonance circuit having an inductor and a capacitor connected in parallel, and at least one grounded capacitor provided between at least one end of the LC parallel resonance circuit and ground. The inductor of the LC parallel resonance circuit is defined by an inductor electrode incorporated in a laminate, the laminate being defined by laminating a plurality of dielectric layers. The capacitor of the LC parallel resonance circuit is defined by a first capacitor electrode and a second capacitor electrode. The first and second capacitor electrodes being incorporated in the laminate. The grounded capacitor is defined by a first ground electrode, a second ground electrode, and a capacitor electrode provided between the first and second ground electrodes within the laminate.
Preferably, the at least one grounded capacitor includes first and second grounded capacitors provided at the ends of the LC parallel resonance circuit. The first grounded capacitor is preferably defined by the first ground electrode, the second ground electrode, and a third capacitor electrode provided between the first and second ground electrodes. The second ground capacitor is preferably defined by the first ground electrode, the second ground electrode, and a fourth capacitor electrode provided between the first and second ground electrodes.
The second capacitor electrode may partially overlap the first ground electrode via a dielectric layer, and the first grounded capacitor may also be defined by the second capacitor electrode and the first ground electrode.
The first capacitor electrode may have a portion that overlaps with the first ground electrode without interposition of the second capacitor electrode, and the second grounded capacitor may also be defined by the overlapping portion of the first capacitor electrode and the first ground electrode.
Preferably, the inductor electrode is provided on a dielectric layer opposite to the dielectric layers on which the grounded capacitor is provided, with respect to the dielectric layers on which the first and second capacitor electrodes are provided.
The inductor electrode, the first capacitor electrode, the second capacitor electrode, the first ground electrode, and the second ground electrode may be provided on different dielectric layers.
Accordingly, a low-pass filter according to preferred embodiments of the present invention allows a grounded capacitor to be defined by two ground electrodes and a capacitor electrode held between the two ground electrodes. Therefore, the capacitances of an ungrounded capacitor (i.e., a capacitor of an LC parallel resonance circuit) and the grounded capacitor are independently controlled.
In another preferred embodiment of the present invention, a mobile communication device including a low-pass filter in accordance with other preferred embodiments of the present invention is provided. The mobile communication device including a low-pass filter which facilitates control of the pass band provides improved performance for transmission and reception.
Other features, characteristics, elements and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.